BRUSSELS — Bombs packed with nails terrorized Brussels on Tuesday in the deadliest assault on the European heartland since the Islamic State’s attacks on Paris four months ago, hitting the airport and subway system in coordinated strikes that were also claimed by the militant extremist group.

The bombings paralyzed Brussels, the headquarters of the European Union and NATO, prompted international travel warnings to avoid Belgium and reverberated across the Atlantic to the United States, where New York and other major cities raised terrorism threat levels. Anxieties intensified about the inability to prevent mass killings at relatively unprotected places.

At least 30 people were killed by two blasts at the Brussels airport departure area around 8 a.m. and one in a subway station shortly after 9. The police found at least one other unexploded bomb in a search of a Brussels house hours later.

And Europe’s most wanted person suddenly became an unidentified man in a white coat and dark hat seen pushing a luggage cart in an airport surveillance photo taken just before the bombings. Two other men in the photo, each wearing a black glove on his left hand, were identified by Belgian prosecutors as suspected suicide bombers who appeared to have died in the explosions.

“To those who have chosen to be the barbaric enemies of liberty, of democracy, of fundamental values, I want to say with the greatest strength that we will remain assembled and united,” the Belgium prime minister, Charles Michel, said at a news conference Tuesday evening, declaring a three-day mourning period.

Francis Vermeiren , the mayor of Zaventem, the Brussels suburb where the airport is located, was quoted by Agence France-Presse late Tuesday as saying all three men had arrived in a taxi, putting suitcases that contained the bombs on luggage carts.

CNN reported on Tuesday night that the police removed bags of evidence from an apartment in the northeast Brussels neighborhood of Schaerbeek, after a taxi driver who saw the photograph of the men told the authorities that he had taken them from the building to the airport that morning, with many large bags.

Passengers who had been in line at airport departure counters described sudden panic and mayhem as the explosions turned the area into a death trap with flames, smoke, flying glass, nails and shrapnel, leaving at least 10 people dead.

“We heard a big noise and saw a big flash,” said one passenger, Ilaria Ruggiano, who had been traveling with six others, including her mother. “My mother went to the floor — she was hit. I just dropped my luggage and went to the floor. A kid came out, bleeding a lot. I tried to help him with a tissue, but it was not enough. There were two bombs.”

The airport was closed, disrupting and diverting dozens of flights and leaving hundreds of passengers stranded, and the Belgian authorities placed the entire metropolitan area on emergency lockdown. It was not clear when the airport would reopen; the Belgian authorities said it was certain to remain closed Wednesday because of the investigation.

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An injured man on the floor of the Brussels airport after the explosions on Tuesday.CreditKetevan Kardava/Associated Press

Then at 9:11 a.m. — the timing may just have been an eerie coincidence — a bomb tore through a car in the rear part of a subway train pulling out of the busy Maelbeek station at the height of the morning rush, killing at least 20 people.

“We felt a boom; we felt the building tremble,” said Henk Stuten, 50, who works for the European Commission in an office above the station. “We saw through the windows that people were rushing out of the metro exit.”

More than 230 people, including people from around the world, were wounded in the three blasts.

In the afternoon, Amaq, a news agency affiliated with the Islamic State, issued a bulletin claiming responsibility for the attacks, calling them the work of suicide bombers.

Frédéric Van Leeuw, the Belgian federal prosecutor, said at a news conference on Tuesday night that “at this stage, it is not possible to draw a formal link with the Paris attacks.” A cell of 10 operatives, a number of them from the Brussels district of Molenbeek, were implicated in the Paris attacks on Nov. 13, which left 130 people dead. The Brussels strikes came only a few days after the Belgian police captured Salah Abdeslam, the only suspect in the Paris assaults believed to have survived, who is considered a potential trove of information.

The State Department on Tuesday warned Americans traveling in Europe to “exercise vigilance when in public places or using mass transportation.” Terrorist groups, the department travel alert said, “continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants and transportation.”

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The Belgian prosecutor Frédéric Van Leeuw said the explosion at the Brussels airport on Tuesday morning was probably a suicide attack, and Prime Minister Charles Michel said Belgium had anticipated a terrorist attack.CreditCreditLaurent Dubrule/European Pressphoto Agency

The threat of further bombings was underscored by the official warnings for people in Brussels to remain indoors, as an intensive search was underway by the police in the Brussels area into Tuesday evening. The federal prosecutor’s office said in a statement that one of the searches, in Brussels’s Schaerbeek district, led to the discovery of “an explosive device containing nails, among other things.” The statement said “chemical products and a flag of the Islamic State” also had been found there.

Late Tuesday, the Belgian Federal Police released new photographs of the suspected suicide bombers and asked people to contact the agency if they recognized them. The public call suggested that whatever information investigators had gathered at the scene, such as DNA, had not yet yielded information allowing them to identify the men or they were unknown to the Belgian authorities.

The heightened security in Belgium extended to two nuclear plants, Doel and Tihange, where nonessential workers were sent home, although the plants remained operational. Ine Wenmaekers, a spokesman for the Belgian nuclear regulatory agency, said that the step was precautionary and that “there was no direct threat to the power plants.”

World leaders reacted with horror and calls for solidarity, though the attacks also spotlighted the fractious debate over terrorism and Islam in Europe and in the American political campaign. The Eiffel Tower and the Burj Khalifa in Dubai were among the world landmarks lit up in the black, red and yellow of Belgium’s flag as night fell.

“Through the Brussels attacks, it is the whole of Europe that is hit,” President François Hollande of France declared. He vowed “to relentlessly fight terrorism, both internationally and internally.”

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Passengers evacuating Brussels’s main airport after the bombings on Tuesday.CreditSylvain Lefevre/Getty Images

The French government ordered 1,600 extra police officers to patrol the nation’s borders, including at train stations, airports and ports. Prime Minister David Cameron of Britain called an emergency meeting of ministers. Foreign Minister Frank-Walter Steinmeier of Germany said the attacks “aim at the heart of Europe.” Pope Francis expressed condolences.

President Obama, speaking in Havana, called the Brussels attacks “yet another reminder that the world must unite, we must be together, regardless of nationality or race or faith, in fighting against the scourge of terrorism.”

But a Russian official tempered sympathy with a scolding of his European colleagues over their policies on migration and terrorism. “It is time for Europe to understand where the real threat is coming from, and to unite its efforts with Russia,” Aleksei K. Pushkov, the chairman of the foreign affairs committee in Parliament, wrote on Twitter.

Since the Paris attacks, security experts have warned that Europe was likely to face additional assaults by the Islamic State and by other militant groups.

The Paris attacks showed that the scale and sophistication of the Islamic State’s efforts to carry out operations in Europe were greater than first believed, and analysts have pointed to Europe’s particular vulnerabilities. They include the huge flow of undocumented migrants from the Middle East last year; the unimpeded movement of European citizens between their home countries, neighboring countries and Syria to fight with the Islamic State; and persistent problems with intelligence-sharing among European countries and even between competing security agencies in some nations.

Few countries have been more vulnerable than Belgium. Among European countries, Belgium has the highest proportion of citizens and residents who have traveled to Syria or Iraq, insular Muslim communities that have helped shield jihadists, and security services that have had persistent problems conducting effective counterterrorism operations, not least in their four-month effort to capture Mr. Abdeslam.

Photographs and amateur video posted online showed the Brussels airport passengers covered in blood and soot, looking stunned but conscious. Some passengers were seen being taken away on luggage carts.

Jérôme Delanois said he was at an Internet cafe near the Delta Air Lines counter when he heard a thunderous noise. “There were two explosions — one big one and one little one,” he said. “The first one blew all the walls and everything. There were burning flames. The first one was bigger. It blew out all the windows.”

Most of the wounded in the subway blast were evacuated to the Rue de la Loi, outside the station, which serves the area that hosts most of the European Union’s core institutions.

Brian Carroll, 31, a communications consultant from Washington, said he was on a subway car near Maelbeek en route to a conference in downtown Brussels when he heard a loud blast.

“As we were pulling into the station, there was suddenly a loud explosion,” he said in a phone interview. “There was smoke everywhere. Everyone dropped to the ground. People were screaming and crying.”

Mr. Carroll said he had remained on the ground for one or two minutes, then got up, pried open a door of the subway car with his hands and fled.

“I thought to myself, ‘I’ve got to get out of here,’ ” he said. “I headed toward an exit. There was smoke and soot everywhere. There was glass everywhere. It was like running through a cloud of dust. I saw the exit of the station was destroyed. I ran out of the station; I ran as far as I could.”

Reporting was contributed by Dan Bilefsky and James Kanter from Brussels; Prashant Rao from London; Lilia Blaise, Nicola Clark, Benoît Morenne and Milan Schreuer from Paris; Rukmini Callimachi and Rick Gladstone from New York; and Julie Hirschfeld Davis from Havana.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Brussels Attacks Shake European Security. Order Reprints | Today’s Paper | Subscribe